1. Field of the Invention
The present invention relates to surface acoustic wave apparatuses, such as a longitudinally-coupled-resonator-type surface acoustic wave filter in which a plurality of interdigital transducers (hereinafter called IDTs) each having a plurality of electrode fingers are arranged in a direction in which a surface acoustic wave propagates, and also relates to communication apparatuses including such surface acoustic wave apparatuses.
2. Description of the Related Art
Surface acoustic wave filters (surface acoustic wave apparatuses) are widely used as bandpass filters in RF stages of portable telephones. The performance required for the bandpass filters includes a low loss, a high attenuation, and a wide bandwidth. Many surface acoustic wave filters satisfying the performance have been invented.
Among these surface acoustic wave filters, as a method for extending the bandwidth of a longitudinally-coupled-resonator-type surface acoustic wave filter, a method has been widely used in which the arrangement of resonant modes is optimized by a condition in which electrode fingers are arranged periodically through two adjacent IDTs, more specifically, by shifting the distance between the centers of adjacent electrode fingers positioned across the space between two adjacent IDTs from half the wavelength determined by the pitch of electrode fingers, such as that disclosed, for example, in Japanese Unexamined Patent Publication No. 05-267990.
In the conventional method, although the bandwidth of a filter can be extended, an insertion loss deteriorates.
In other words, when the distance between adjacent electrode fingers positioned across the space between two adjacent IDTs is shifted from half the wavelength determined from the pitch of electrode fingers, periodic continuity deteriorates in a propagation path of a surface acoustic wave at that portion. Especially in a surface acoustic wave filter including a piezoelectric substrate which uses a leaky surface acoustic wave, such as a 36-degree Y-cut X-propagation LiTaO3 substrate or a 64-degree Y-cut X-propagation LiNbO3, a loss caused by the emission of a bulk wave increases. As a result, although the bandwidth of the filter is extended, an insertion loss (propagation loss) deteriorates.
In order to overcome the problems described above, preferred embodiments of the present invention provide a longitudinally-coupled-resonator-type surface acoustic wave apparatus which has a wide pass bandwidth, has a small insertion loss and a small VSWR, and does not cause deterioration of the dielectric strength, and for which electrodes are easily formed, and to provide a communication apparatus which includes such a novel surface acoustic wave apparatus.
According to one preferred embodiment of the present invention, a surface acoustic wave apparatus of a longitudinally-coupled-resonator type includes a plurality of interdigital transducers each having a plurality of electrode fingers, arranged along the propagation direction of a surface acoustic wave on a piezoelectric substrate, wherein the pitch of an electrode finger located in an end portion facing another interdigital transducer is smaller than that of electrode fingers at the other portions in at least one interdigital transducer, and the duty (also called a metallization ratio) of the electrode finger of which the pitch is made smaller is smaller than that of the electrode fingers at the other portions.
According to the above-described unique structure, in a longitudinally-coupled-resonator-type surface acoustic wave filter having a plurality of interdigital transducers, the pitch of an electrode finger located in an end portion where two IDTs face each other is smaller than that of electrode fingers at the other portions in the IDT, and the duty of the electrode finger (hereinafter called a narrow-pitch electrode finger) of which the pitch has been made smaller is smaller than that of electrode fingers at the other portions. Therefore, the filter has a bandwidth that is substantially equal to or wider than that obtained by a conventional technology, has a small insertion loss, does not cause deterioration of the dielectric strength, and its electrodes are easily formed.
In the above-described surface acoustic wave apparatus, it is preferred that both IDTs adjacent to each other be provided with an electrode finger having a different pitch, and the distance between the centers of adjacent electrode fingers positioned across the space between two IDTs be made substantially equal to about 0.5xcexI2, where xcexI2 indicates the wavelength determined by the pitch of the electrode fingers of which the pitch has been made different, and xcexI1 indicates the wavelength determined by the pitch of the other electrode fingers.
According to the above-described unique structure, when both IDTs adjacent to each other are provided with an electrode finger having a different pitch, the distance between the centers of adjacent electrode fingers positioned across the space between two IDTs is made almost equal to 0.5xcexI2. Therefore, since the continuity of a surface acoustic wave propagating between the IDTs is ensured, the deterioration of an insertion loss is reliably prevented.
In the above-described surface acoustic wave apparatus, it is further preferred that one of IDTs adjacent to each other be provided with an electrode finger having a different pitch, and the distance between the centers of adjacent electrode fingers positioned across the space between two IDTs be made almost equal to 0.25xcexI1+0.25xcexI2, where xcexI2 indicates the wavelength determined by the pitch of the electrode finger of which the pitch has been made different, and xcexI1 indicates the wavelength determined by the pitch of the other electrode fingers.
According to the above-described unique structure, when one of IDTs adjacent to each other is provided with an electrode finger having a different pitch, the distance between the centers of adjacent electrode fingers positioned across the space between two IDTs is made almost equal to 0.25xcexI1+0.25xcexI2. Therefore, since the continuity of a surface acoustic wave propagating between the IDTs is ensured, the deterioration of an insertion loss is reliably prevented.
In the surface acoustic wave apparatus, it is further preferred that, in the interdigital transducer in which the pitch of the electrode finger located in the end portion facing the another interdigital transducer is made different from that of the other electrode fingers in the interdigital transducer, the distance between the centers of adjacent electrode fingers one of which is an electrode finger of which the pitch has been made different and the other of which is an electrode finger of which the pitch has not been made different be made almost equal to 0.25xcexI1+0.25xcexI2.
According to the above-described unique structure, since the distance between the centers of adjacent electrode fingers one of which is an electrode finger of which the pitch has been made different and the other of which is an electrode finger of which the pitch has not been made different is made almost equal to 0.25xcexI1+0.25xcexI2, the continuity of a surface acoustic wave propagating between the electrode finger of which the pitch has been made different and the electrode finger of which the pitch has not been made different is ensured, and the deterioration of an insertion loss is prevented.
The surface acoustic wave apparatus may be configured such that the duty of an electrode finger of which the pitch has been made smaller is smaller than that of electrode fingers in the other portions in the IDT and is substantially equal to or larger than about 0.45. According to the above-described unique structure, since the duty of an electrode finger of which the pitch has been made smaller is smaller than that of electrode fingers in the other portions in the IDT and is substantially equal to or larger than about 0.45, the suppression of the deterioration of an insertion loss is even more reliably achieved.
The advantages described above are achieved in another aspect of preferred embodiments of the present invention through the provision of a surface acoustic wave apparatus of a longitudinally-coupled-resonator type, having a plurality of IDTs each having a plurality of electrode fingers arranged along the propagation direction of a surface acoustic wave on a piezoelectric substrate, wherein the duty of adjacent electrode fingers positioned across the space between at least any two IDTs is smaller than that of electrode fingers at the other portions.
According to the above-described unique structure, since the duty of adjacent electrode fingers positioned across the space between at least any two IDTs is smaller than that of electrode fingers at the other portions, the apparatus has a wide pass band, suppresses the occurrence of electrostatic destruction, and matching in a manufacturing process can be easily implemented.
The surface acoustic wave apparatus may be configured such that the distance between the centers of adjacent electrode fingers positioned across the space between two IDTs is smaller than that (electrode-finger pitch) between the centers of electrode fingers in the other portions. The surface acoustic wave apparatus may have a balanced-unbalanced input and output.
The foregoing advantages are achieved in still another aspect of preferred embodiments of the present invention through the provision of a communication apparatus using any of the above-described surface acoustic wave apparatuses.
According to the above-described unique structure, a surface acoustic wave apparatus having a filter element or a balanced-to-unbalanced conversion function can be made compact especially in a GHz band equal to and higher than 100 MHz. Therefore, the communication apparatus including the surface acoustic wave apparatus can also be made compact. Further, since the used surface acoustic wave apparatus can be made to have a low loss, as described above, the circuit structure can be simplified. Also with this point, the communication apparatus can be made compact.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.